The invention concerns a skull trepanation drill (bone drill) with a drive shaft connectable to a drive mechanism, with a sleeve-shaped housing surrounding the drive shaft and rotatably mounted on the latter, provided at its free end with a ring-shaped auxiliary cutter, with a main cutter mounted as an extension of the drive shaft in the interior of the housing in an axially displaceable manner, protruding beyond the auxiliary cutter, which can be connected in a rotationally rigid manner with the housing by means of a coupling element protruding beyond its circumference and engaging a recess on the inside of the housing, with a spring which forces the main cutter in the axial direction away from the drive shaft, with coupling elements on the end surface of the drive shaft and the opposing end surface of the main cutter for the purpose of rotationally coupling the main cutter and the drive shaft by mutual engagement when the main cutter is forced in the axial direction against the drive shaft, with at least one locking body mounted in the wall of the drive shaft and free to slide in the radial direction, which is forced into a first position by means of a locking bolt which is axially displaceable inside the hollow drive shaft and has at least one recess in its circumference and which protrudes beyond the circumference in its first position, thereby engaging into a recess on the inside of the sleeve-shaped housing, thereby securing the housing against axial displacement along the drive shaft, and which can be slid into a second position in which the recess in its circumference receives the locking body in such a manner that it no longer protrudes beyound the circumference of the drive shaft and thereby permits axial displacement of the housing on the drive shaft, and with a spring which slides the locking bolt into the first position.
Skull trepanation drills of this design have been exceptionally successful as so-called automatic skull trepanation drills, since the cutters are automatically decoupled from the drive shaft after completion of the skull boring, thereby preventing the danger of damage to the brain mass below the drilled skull bone. Skull trepanation drills of this type comprise a number of individual parts, and it is necessary to dis-assemble these individual parts completely for purposes of cleaning.
For this purpose the locking mechanism described above was found very suitable, in which the housing carrying the auxiliary cutter mounted on the drive shaft is secured to the drive shaft in the axial direction by a locking mechanism which comprises at least one locking body engaging a recess on the inside of the housing and free to slide in the radial direction in the outer wall of the drive shaft. To effect the radial displacement of the locking body there is provided in the hollow interior of the drive shaft, in one previously known skull trepanation drill, an axially displaceable locking bolt, which is held by means of a compression spring in a position in which locking bodies bear against its circumference and thereby protrude beyond the circumference of the drive shaft and engage into a recess on the inside of the housing. The locking bolt can be displaced against the action of the spring in the axial direction in such a manner that the locking bodies in the wall of the drive shaft are free to enter a recess in the circumference of the locking bolt, so that the locking bodies liberate the housing and permit its axial motion along the drive shaft.
In previously known skull trepanation drills of this design the displacement of the locking bolt inside the hollow drive shaft was achieved by displacing the locking bolt against the action of the spring by means of a pin through a concentric hole in the drive shaft. This design has not been found expedient in practice because it requires a separate tool. This involves the danger, particulary during assembly of the drill, that the sterile parts of the drill will be contaminated by this tool. Moreover, the proper tool must be available at the required instant, and this may also lead to problems in practice.
It is the object of the present invention to improve a skull trepanation drill of the type described above in such a manner that locking and unlocking of the individual parts is possible without the use of a separate tool. According to the invention, this task is solved in a skull trepanation drill of the design described above in that for axial displacement of the locking bolt the latter contacts a follower which slides in the drive shaft in the axial direction and by means of a cam surface which is secured against axial displacement along the drive shaft but is free to rotate about the longitudinal axis of the drive shaft, so that the follower bears against the cam surface and is displaced in the axial direction when the cam surface is rotated.
This design makes it possible in the simplest manner, by rotating the cam surface with respect to the drive shaft, to retract or extend the locking bodies in the drive shaft without the need for a separate tool.
It is expedient if the follower comprises a pin passing diametrically through the drive shaft and guided by two elongated holes in the wall of the drive shaft which are oriented parallel to its axis.
In the preferred embodiment of the invention it is provided that the cam surface bears in the axial direction under the pressure of the spring which slides the locking bolt against a stop attached to the drive shaft, which preferably comprises a ring flange.
It is also expedient if both the drive shaft and the cam surface are provided with a knurled flange so as to facilitate the relative displacement of the cam surface with respect to the drive shaft.
Additional expedient configurations of the invention constitute objects of the sub-claims.